Electrical equipment such as audio-visual equipment or PCs and mobile communication devices have rapidly become portable and cordless in recent years. Aqueous electrolyte batteries such as nickel cadmium or nickel metal hydride batteries have been mainly used as the drive power source of these electrical equipment in the past, but these batteries have been replaced recently by non-aqueous electrolyte batteries, typically represented by lithium rechargeable batteries that can be charged rapidly and have high volume/weight energy densities. The nickel cadmium or nickel metal hydride batteries, on the other hand, are being more and more specifically used for power tools or electric vehicles that need a drive power source with high load characteristics.
The trend in the non-aqueous electrolyte battery systems is towards a prismatic design because it can achieve high energy density, excellent load characteristics, and high space utilization, and is advantageous in making the equipment thinner. Furthermore, these batteries must meet the demands for higher voltage and capacity, as the portable electrical equipment have more and more advanced performance and functionality. Electrode assemblies conventionally used in prismatic batteries are mostly laminated types, having strips of stacked positive and negative electrode plates with separators interposed therebetween, and they have relatively low heavy load characteristics and cannot accept a rapid charge because of the small electrode reaction area. Accordingly, to meet the above demands, non-aqueous electrolyte batteries constructed with wound electrode assemblies of a prismatic shape have widely been used, in which laminated positive and negative electrode plates, and separators therebetween, are wound around. This prismatic wound electrode group has a construction in which a large number of positive and negative electrode plates are laminated when viewed in a longitudinal cross section, and thus can achieve large electrode reaction area.
The above wound electrode group of prismatic shape is manufactured by winding positive and negative electrode plates around a core that has a circular or elliptic cross section, and by compressing the wound assembly so that its circular or elliptic cross section becomes prismatic; with this method, however, the electrodes are often misaligned or brought out of contact near bent portions on the inside, whereby the electrode reaction is made uneven, leading to deterioration or variation of battery capacity.
For this reason, there has been a method for manufacturing a prismatic wound electrode group in which positive and negative electrode plates are wound around a flat plate core so that the wound assembly has an oblong cross section, as disclosed in Japanese Patent Laid-Open Publication No. 6-96801; or a method for manufacturing a prismatic wound electrode group in which positive and negative electrode plates are wound around a core that has substantially a rhombic cross section, and compressed so that the wound assembly has a prismatic cross section, as disclosed in Japanese Patent Laid-Open Publication No. 8-171917. In yet another prior art method for manufacturing a prismatic wound electrode group, positive and negative electrode plates are wound into a shape having a prismatic cross section and compressed with high pressure, as disclosed in Japanese Patent Laid-Open Publication No. 10-302827.
Cylindrical wound electrode assemblies used for cylindrical batteries, on the other hand, are manufactured by a method disclosed, for example, in Japanese Patent Laid-Open Publication No. 60-180071, in which, after winding positive and negative electrode plates into a shape that has a circular cross section, an adhesive tape is tightly wound around the electrodes so that the assembly has an outside diameter slightly smaller than the inside diameter of the cylindrical battery case in which it will be accommodated. When accommodating this cylindrical wound electrode group into the battery case, while it is held with a pinch tool on the outside, part of the adhesive tape and separator is cut off by a cutter and the lower part of the assembly is inserted into the case; the assembly can then be inserted into the case without removing the adhesive tape, with the electrodes loosened due to their reaction forces. Easy accommodation of the electrode group into the battery case and reduction in the risk of short circuits are thus achieved.
Japanese Patent Laid-Open Publication No. 10-64577 discloses another method for manufacturing a wound electrode group used for a cylindrical battery, in which, before winding the positive and negative electrode plates, an alkaline resistant material is attached to one or both faces of the positive electrode plate in the winding start part so as to reduce the amount of the separator to achieve a lower package ratio (ratio of battery parts other than active parts to the entire volume of the battery) and better assemblability and higher capacity, and to prevent short circuits resulting from cracks that can be formed at the end face of the positive electrode plate in the winding process.
Despite this attempt to reduce the risk of short circuits caused by cracks at the winding start in the cylindrical wound electrode group of Japanese Patent Laid-Open Publication No. 10-64577, crack-induced short circuits can still occur when the positive and negative electrode plates are thick to a certain extent, because the cross section of the wound assembly will be polygonal and not circular at the winding start. This trouble may be prevented in the cylindrical wound electrode group of Japanese Patent Laid-Open Publication No. 60-180071 by the adhesive tape tightly wound around the assembly, but the battery capacity is lowered by the use of adhesive tape because of decreased package ratio of the battery, and moreover, when inserting the assembly into the battery case, the assembly loosens more than required when part of the adhesive tape and separator is cut, creating gaps between adjacent positive and negative electrode plates, whereby the electrode reaction is made uneven, leading to deterioration or variation of battery capacity.
The recent trend in the wound electrode assemblies is towards thinner positive and negative electrode plates and separators and larger number of winding turns to achieve higher capacity so that the assemblies can be used for power tools or electric vehicles that require a drive power source with high load characteristics. In respect of achieving higher capacity, the wound electrode assemblies shown in Japanese Patent Laid-Open Publication Nos. 60-180071 and 10-64577 are not suitable because looseness exists in the assembly; the adjacent positive and negative electrode plates must be in tight contact with each other and wound around without any looseness or misalignment, as in the manufacturing methods of wound electrode assemblies shown in Japanese Patent Laid-Open Publication Nos. 6-96801, 8-171917, and 10-302827. This is because gaps between adjacent positive and negative electrode plates make the electrode reaction uneven and lead to deterioration or variation of battery capacity.
While cracks at the winding start can be reduced by retaining the tightly wound state of the assembly with adjacent positive and negative electrode plates in tight contact with each other as in the inventions disclosed in Japanese Patent Laid-Open Publication Nos. 6-96801, 8-171917, and 10-302827, there are the following problems: Firstly, the tightly wound assembly of thin positive and negative electrode plates and separators can hardly be impregnated with electrolyte in the electrolyte injecting process in the manufacture of the battery, and the charge/discharge reaction of the complete battery may become uneven and high load characteristics may be deteriorated because of the uneven impregnation of electrolyte.
Secondly, after the prismatic wound electrode group is accommodated in the battery case with electrolyte and functions as a battery, the active materials of the positive and negative electrode plates expand due to repeated charge and discharge, because of which large buckling occurs in the linear portions between curved portions at both lengthwise ends, as shown in FIG. 9, which shows a conventional prismatic wound electrode group 50 constructed and used as a battery. While the linear portions of the oval cross section of the positive and negative electrode plates stretch along the length, the curved portions at both ends are restricted from outward expansion by the battery case and can hardly stretch; in addition, the assembly is fixed so as to remain tightly wound wherein the positive and negative electrode plates make tight contact with each other; thus the buckling in the linear portions is inevitable.
This buckling causes large gaps particularly in the center of the prismatic wound electrode group 50, whereby the positive and negative electrode plates separate from each other, making the electrode reaction uneven and lowering the charge/discharge cycle characteristics. Moreover, a buckling deformation may damage the separator and cause short circuits across the positive and negative electrode plates. Further troubles caused by the buckling include an outward bulge of the wound electrode group 50 in the central portion, abnormal deterioration of the battery because of a large deformation from the predetermined battery case dimensions, and the possibility of electrical disconnection of the battery from the battery holder case of the equipment because of the deformation of the battery case.
As for the cylindrical wound electrode assemblies, the cross section of the wound assembly can be made circular at the winding start if the positive and negative electrode plates are thin enough, but because the active materials of the positive and negative electrode plates expand due to repeated charge and discharge, an inward buckling deformation tends to occur in the central portion of the circular cross section, as shown in FIG. 10, which shows a conventional cylindrical wound electrode group 60 constructed and used as a nickel metal hydride battery. As noted above in relation to the prismatic wound electrode group 50, such buckling makes the electrode reaction uneven and causes various troubles such as deterioration of charge/discharge cycle characteristics and short circuits across the positive and negative electrode plates due to the damaged separator. The above problem found in the cylindrical wound electrode group 60 occurs also in a wound electrode group that has a substantially square cross section with four rounded corners.
The present invention has been devised in view of the above problems encountered in prior art, its object being to provide a battery having a wound electrode group which is impregnated with electrolyte swiftly and evenly while the positive and negative electrode plates and separator are tightly in contact with each other, and in which buckling is suppressed, and a suitable method for manufacturing the wound electrode group used for this battery.